The invention relates to a method for operating an internal combustion engine wherein at partial engine load a lean base mixture of air and fuel is formed in the engine combustion chamber and ignited by compression ignition, and at full load a stoichiometric mixture is formed and ignited by spark ignition.
DE 195 19 663 A1 discloses a method for operating an internal combustion engine with spontaneous ignition, in which in a first stage a homogenous, pre-compressed fuel/air mix which is not suitable for spontaneous ignition is provided in the combustion chamber of the internal combustion engine, and in a second stage an additional quantity of the same fuel is injected into the working space in order to bring about the spontaneous ignition. In this case, the fuel/air mix is formed externally and introduced into the engine cylinder, where it is compressed close to the spontaneous ignition point. The injection of the additional quantity of fuel in the second stage takes place in finely atomized form, avoiding contact with wall, so as to form a mix cloud in which, on the one hand, the fuel/air ratio is no greater than the stoichiometric mixing ratio and in which, on the other hand, the spontaneous ignition is achieved.
Furthermore, DE 198 52 552 C2 discloses a method for operating a four-stroke internal combustion engine which, at part-load, forms a lean base mix from air, fuel and retained exhaust gas and, at full load, forms a stoichiometric mix. At part load, compression ignition takes place, whereas at full load spark ignition takes place. Furthermore, mechanically controlled exhaust-gas retention with switchable valve closure overlap and exhaust gas throttling is provided. An activation fuel amount may be injected into the retained exhaust gas. The quantity of retained exhaust gas, with the valve closure overlap switched on, is controlled or preset as a function of the engine speed and engine load by an exhaust-gas throttle valve which is active for all the combustion chambers. Upon opening of the intake valves of the individual cylinders the pressure in the various combustion chambers is equalized by a cylinder-selective, cycle-consistent activation injection.
A method for operating a four-stroke, reciprocating-piston internal combustion engine is also known from DE 198 18 569 C2. It is characterized by a homogenous, lean base mix of air, fuel and retained exhaust gas and by compression ignition and direct injection of the fuel into the combustion chamber. The volume of the combustion chamber changes cyclically. The combustion chamber can be filled with fresh gas through at least one intake valve, while the combustion exhaust gases can be at least partially expelled through at least one exhaust valve. In the part-load range and in the lower full-load range, the internal combustion engine is operated with compression ignition and preferably mechanically controlled exhaust-gas retention, whereas in the full-load range and high part-load range it is operated by spark ignition.
One drawback of the methods disclosed in the above-mentioned documents is in particular that the temperature of the exhaust gas and the composition of the working gas change when the engine speed changes. The reactivity, that is the ignitability of the mix during compression ignition, is likewise altered as a result even to the extent of causing misfires if the operating gas temperatures are too low.
It is therefore an object of the invention to provide a method for operating an internal combustion engine in which changes in the reactivity, that is, the ignitability of the mix in the event of changes in engine speed can be taken into account and/or corrected.